1. Field of the Invention
The present invention relates to a self-etching and self-priming bonding composition for tooth restoration that keeps the restorative dental material bonded to the defective tooth for a long period of time.
2. Description of the Related Art
In the last half of the 20th century, the technology of bonding an acrylic resin to teeth had a great influence on the development of dental treatment. The first technique referred to is a technique developed by M. Buonocore in 1956. It comprises etching and roughening of the enamel of a tooth with an aqueous solution of phosphoric acid, resulting in a significant increase in the area of the bonding interface and an increase of the bonding strength between the tooth and the resin.
Later, various types of functional monomers capable of forming chemical bonds with the components of teeth have been developed. With those, the technique of bonding resins to the dentin of teeth has made remarkable progress in dental treatment. The proposed functional monomers include, for example, polymerizable monomers having an acid group, such as a phosphoric acid group or a carboxylic acid group, having high reactivity with the calcium hydroxyapatite (HAP) of teeth. Furthermore, other monomers capable of forming covalent bonds to the collagen of teeth have been proposed.
In Japanese Patent Laid-Open No. 45510/1985 the present applicant proposed a remarkable technique, wherein water is added to a dental bonding composition that contains a polymerizable monomer having a phosphoric acid residue. The dental bonding composition further comprises a specific terpolymerization initiator selected from diacyl peroxides, amine compounds, arylsulfinic acid salts, water, and optionally any other copolymerizable monomer. Conventionally it is assumed that the influence of water on the restored area of teeth is to be minimized to ensure good adhesion of a restorative material to teeth that are in constant contact with saliva. However, the disclosed invention is based on the finding that the bonding strength of the dental composition can be enhanced by adding water. The inventors disclose in the specification of 45510/1985 that the dental bonding composition exhibits high bonding strength even to the teeth not previously etched with an acid. The bonding strength may be further enhanced if the teeth are etched with an acid before the composition is applied. However, a self-etching or self-priming dental bonding composition is not disclosed.
After that, it was found that primers, if applied to teeth before the bonding composition, could enhance the bonding strength of the compositions to a great extent. Various proposals have been made for primers. Most of them ensured a complete wetting of the teeth by the bonding compositions. For example, Japanese Patent Laid-Open No. 223289/1987 discloses a primer for hard tissue which comprises an acid, a water-soluble film-forming agent, and optionally water. A specific acid is used in the invention. Its salt with calcium must be soluble in the water-soluble film-forming agent for the primer to provide high bonding strength. The specification discloses that the initial bonding strength of the bonding composition to the dentin of the tooth is increased if a primer is first applied to a tooth followed by the application of a bonding composition. However, when the bonded tooth sample is kept in water for a long period of time, the bonding strength of the bonding composition decreases, and the bonding durability is not satisfactory. Thus, the proposed primer has a drawback.
Japanese Patent Laid-Open No. 240712/1991 discloses a primer composition that comprises water, a polymerizable compound having a hydroxyl-group, a polymerizable compound having an acid group, and a curing agent. The object of the proposed primer is to provide high bonding strength and bonding durability. However, the polymerizable compound having an acid group is not specifically defined, and includes the polymerizable compounds that are exemplified as acids in the specification of above-mentioned 223289/1987.
The two techniques disclose a primer treatment of teeth instead of the ordinary etching with phosphoric acid to provide high bonding strength. However, bonding treatment with a liner or a similar bonding composition after the primer treatment is required when adhesive resin cement is used. Accordingly, the proposed techniques are still problematic because the use of primers does not shorten the process of dental treatment and does not fully meet the requirements of users and dentists.
The inventor has studied the mechanism of the bonding behavior and the bonding durability of the bonding compositions of above prior art. He found that the dentin of teeth is a complicated bio-tissue that comprises collagen fibers and HAP crystals. When an acidic primer having an acid ingredient is applied to dentin, it will partially dissolve the HAP crystals in the dentin. At the same time, the water-soluble monomer ingredient of the primer will penetrate into the collagen fibers remaining in the dentin after the HAP crystals were dissolved. As a result, the modified dentin surface is compatible with a bonding agent. The primer is then cured. The collagen fibers of the dentin will be entangled with the resin formed by polymerization of monomers and water-soluble film-forming agent. The bonding composition is applied to the tooth after the primer. As a result, the bonding composition has good bonding strength to the tooth.
The boundary area between the dentin of the tooth and the resin layer formed from the primer was analyzed. A calcium salt exists around the surface of the HAP crystals in the boundary area. The calcium salt was formed by reaction of above acid with HAP. In addition, resin portions containing the collagen fibers are left as they are after the partial dissolution of HAP. If the calcium salt of the acid is soluble in water it will gradually dissolve when the bonded sample is kept in water for a long period of time. Accordingly, the packing density of the bonding resin around the bonding interface will be lowered. As a result, the strength and durability of the bonding layer will also be lowered.
The present inventor has found the particular advantage of the prior art primers capable of exhibiting high bonding strength even when they are directly applied to teeth that are not previously subjected to acid etching (probably due to the self-etching effect of the primers). The present inventor further recognized the disadvantage of low bonding durability due to the solubility of the calcium salt of the acid in the bonding interface layer in water.
The present invention overcomes the drawback of the prior art primers on the basis of the fundamental knowledge relating to the composition of ordinary bonding compositions for dental use.
One object of the present invention is to provide a self-etching and self-priming bonding composition which ensures good initial bonding strength to teeth and long-lasting durability. The self-etching and self-priming bonding composition is useful in dental treatment and provides good bonding to tooth tissue and does not require any pre-treatment, such as acid etching and priming.
Another object of the invention is to provide a liquid bonding composition with rapid curability.
Still another object of the invention is to provide a bonding composition having the an increased bonding strength, whereby the thickness of the bonding layer can be controlled in any desired manner.
The objects of the invention are attained by providing a bonding composition which comprises:
a mixture of a polymerizable compound having acid group, a water-soluble film-forming agent, water, and a curing agent;
wherein a calcium salt formed from said polymerizable compound having an acid group is insoluble in water;
wherein said film-forming agent is a polymerizable compound; and
wherein said film-forming agent is miscible with a physiological saline solution.
The inventor has found the following:
(1) Acids with a specific acid strength are preferred in bonding compositions for dental use. More preferred are those having a polymerizable group in the molecule that are capable of forming a stable chemical bond to HAP and the amino group of collagen of teeth as well as a stable chemical bond to a water-soluble polymerizable compound to be coated over teeth. Selective use of the specific acid in a bonding composition ensures better bonding durability.
(2) Preferred are those acids of which the calcium salts resulting from acid etching have a lower degree of solubility in water. If the calcium salt of the acid existing around the bonding interface does not dissolve in water, a lowering of the bonding strength of the resin may be prevented.
(3) Preferred are film-forming agents that are soluble in the acid to any desired degree and are copolymerizable with the acid. More preferred are those miscible with the fluid in body tissue, as they could well penetrate into hard tissue.
(4) Preferably, a polymerization initiator is added to bonding compositions. More preferably, polyfunctional monomers and others are added within a range not interfering with the uniformity of the compositions. They promote the polymerization of the bonding layer and make the polymerized bonding layer harder by introducing a cross-linked structure. The bonding strength and durability of the bonding layer are further enhanced.
(5) Preferably, a form-retaining agent is added to bonding compositions. It reinforces the bonding layer and, in addition, provides the desired thickness of the bonding layer. Accordingly, the polymerization inhibition in the bonding interface that may be caused by oxygen in air can be prevented. The bonding interface, may be smoothly polymerized and cured for increased bonding strength of the cured bonding layer.
Based on his studies and the results as above, the inventor prepared samples of a bonding composition to be mentioned below, and tested them for their bonding behavior. The samples were applied to the dentin of bovine teeth and cured. Immediately after curing, the bonding layer had a bonding strength of at least 15 MPa. After the bonded samples were stored in water at 50xc2x0 C. for one month, the bonding strength of the bonding layer was still at least 14 MPa. After the bonding test, the samples were observed, and most of them showed cohesive failure with the bonded tooth broken. The numerical data were obtained as above, but it is believed that the actual bonding strength of the bonding composition should be higher than the level of the data. On the basis of these findings, the inventor has reached and completed the present invention.
The polymerizable compound having an acid group may be soluble in water, or may dissolve in water to form micelles therein. Furthermore, the polymerizable compound may form an emulsion, a dispersion or a suspension. To provide good bonding of the composition, the polymerizable compound having an acid group is preferably as follows: When it is mixed with water to have a concentration of 1% by weight, the pH of the resulting mixture is at most 3, preferably at most 2.5, more preferably falls between 1.8 and 2.5. The pH includes all values, therebetween, especially including a pH of 0.1, 0.5, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7.
The calcium salt of the polymerizable compound having an acid group must be insoluble in water, and substantially insoluble in the film-forming agent. The solubility of the calcium salt of the acid may be determined as follows: A predetermined amount of calcium carbonate is added to a plurality of solutions having varying acid concentrations in water, or to a plurality of solutions having varying acid concentrations in a film-forming agent. 1.05 equivalent of acid is reacted with calcium carbonate to form a calcium salt. Where the reaction solution is clear the calcium salt has dissolved in the solution. However, where it is cloudy, the calcium salt did not dissolve. If the solubility of the calcium salt in water falls between 0.0001 and 0.05 mol/liter (M/L), then the calcium salt is insoluble in water, and satisfies the requirement of the invention. Preferably, the solubility of the calcium salt in water falls between 0.0001 and 0.01 M/L, more preferably between 0.0001 and 0.001 M/L. The solubility includes all values therebetween, and especially including 0.0002, 0.0003, 0.0004, 0.0005, 0.0006, 0.0007, 0.0008, 0.0009, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008 and 0.009 M/L.
Calcium salts of acids that are soluble in water to some degree just after their formation through reaction with teeth but are gradually converted into more stable insoluble salts are preferred compared to those that may be readily formed immediately after contact with teeth, such as salts of sulfuric acid. Preferably, the calcium salt of the acid is soluble to some degree in the acid-containing bonding composition. However, after the acid in the composition has completely reacted with a tooth, the resulting calcium salt is not soluble in the composition that does not contain any acid anymore. If hydrochloric acid, nitric acid, p-toluenesulfonic acid or the like are used in place of the polymerizable compound having an acid group, the solubility of the calcium salt of the acid in water is too high and the composition could not exhibit high bonding strength and durability. Therefore, these acids are unsuitable for the purpose of the present invention. On the other hand, the solubility of the calcium salts of phosphoric acid, oxalic acid or the like in water is too low, and the calcium salt of sulfuric acid crystallizes too rapidly. Therefore, the bonding composition containing any of these acids could not be self-etchable to a satisfactory degree, and the acids are not suitable for the present invention.
The polymerizable compound having an acid group is not specifically limited, as long as it satisfies the above solubility requirements. The following compounds are preferred for further improvement of the initial bonding strength and durability of the bonding composition: vinyl compounds having an acid group, such as a phosphoric acid group or a carboxyl group in the molecule. More preferred are vinyl compounds having a phosphoric acid group in the molecule. The vinyl compounds having a phosphoric acid group in the molecule include, for example, (meth)acryloyloxyalkyl phosphates (referring to acryloyloxyalkyl phosphates and methacryloyloxyalkyl phosphates), such as 2-(meth)acryloyloxyethyl phosphate, 2- or 3-(meth)acryloyloxypropyl phosphate, 4-(meth)acryloyloxybutyl phosphate, 6-(meth)acryloyloxyhexyl phosphate, 8-(meth)acryloyloxyoctyl phosphate, 10-(meth)acryloyloxydecyl phosphate, 12-(meth)acryloyloxylauryl phosphate, 16-(meth)acryloyloxycetyl phosphate, 18-(meth)acryloyloxystearyl phosphate and 20-(meth)acryloyloxyeicosyl phosphate; di(meth)acryloyloxyalkyl phosphates such as 1,3-di(meth)acryloyloxypropyl-2phosphate; (meth)acryloyloxyalkylaryl phosphates such as 2-(meth)acryloyloxyethylphenyl phosphate, 2-(meth)acryloyloxyethylanisyl phosphate and 2-(meth)acryloyloxyethyltolyl phosphate; (meth)acryloyloxyalkylaryl phosphonates such as 2-(meth)acryloyloxyethylphenyl phosphonate; (meth)acryloyloxyalkyl thiophosphates, such as 2-(meth)acryloyloxyethyl thiophosphate, 2- or 3-(meth)acryloyloxypropyl thiophosphate, 4-(meth)acryloyloxybutyl thiophosphate, 6-(meth)acryloyloxyhexyl thiophosphate, 8-(meth)acryloyloxyoctyl thiophosphate, 10-(meth)acryloyloxydecyl thiophosphate, 12-(meth)acryloyloxylauryl thiophosphate, 16-(meth)acryloyloxycetyl thiophosphate, 18-(meth)acryloyloxystearyl thiophosphate and 20-(meth)acryloyloxyeicosyl thiophosphate; di(meth)acryloyloxyalkyl thiophosphates such as 1,3-di(meth)acryloyloxypropyl-2-thiophosphate; (meth)acryloyloxyalkylaryl thiophosphates such as 2-(meth)acryloyloxyethylphenyl thiophosphate, 2-(meth)acryloyloxyethylanisyl thiophosphate and 2-(meth)acryloyloxyethyltolyl thiophosphate; and (meth)acryloyloxyalkylaryl thiophosphonates such as 2-(meth)acryloyloxyethylphenyl thiophosphonate. Also employable are their precursors, which may be hydrolyzed in water to yield the acids. One or more of these may be used singly or in combination. They may be further combined with any other acid compound as long as the combinations attain the object of the invention. Preferred acid compounds are (meth)acryloyloxyalkyl phosphates in which the number of the carbon atoms constituting the alkyl group falls between 6 and 24, and more preferably between 8 and 20. Particularly preferred is 10-methacryloyloxydecyl phosphate (e.g., 10-methacryloyloxydecyl dihydrogenphosphate).
The vinyl compounds having a carboxyl group in the molecule include, for example, (meth)acryloyloxyalkoxycarbonylphthalic acids such as 4-(meth)acryloyloxyethoxycarbonylphthalic acid, 4-(meth)acryloyloxybutoxycarbonylphthalic acid, 4-(meth)acryloyloxyhexyloxycarbonylphthalic acid and 4-(meth)acryloyloxydecyloxycarbonylphthalic acid; (meth)acryloyloxyalkoxyalkoxycarbonylphthalic acids such as 4-(meth)acryloyloxyethoxyethoxycarbonylphthalic acid; dicarboxylic acids such as 11-(meth)acryloyloxyundecyl-1,1-dicarboxylic acid; and their anhydrides capable of being hydrolyzed to yield the acids. One or more of these may be used singly or in combination. They may be further combined with any other acid compound as long as the combinations attain the object of the invention. Preferred compounds are 4-(meth)acryloyloxyethoxycarbonylphthalic acid and 11(meth)acryloyloxyundecyl-1,1-dicarboxylic acid.
The amount of the polymerizable compound having an acid group in the bonding composition of the invention may vary within a range provided the object of the invention is attained. Preferably, however, it falls between 0.1 and 50% by weight, more preferably between 1 and 50% by weight, even more preferably between 2 and 40% by weight of the bonding composition. These amounts include all values therebetween, and especially including 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, 30, 32.5, 35, 37.5, 40, 42.5, 45 and 47.5% by weight.
The film-forming agent differs from the polymerizable compound having an acid group mentioned above. It is a water-soluble polymerizable compound miscible with a physiological saline solution having a concentration of 9 g/L. Physiological saline solution may be a substituent for the fluid in body tissue. A mixture of two or more polymerizable compounds or a mixture of the film-forming agent with the polymerizable compound having an acid group and/or with an organic solvent can be used as the water-soluble film-forming agent. These mixtures can be soluble in water and miscible with a physiological saline solution even though they are not fully soluble in water and not miscible with a physiological saline solution by themselves. The bonding composition containing a film-forming agent of that type can form a uniform solution.
The film-forming agent must be soluble in water. Preferably, at least 5% by weight of the agent are soluble in water. More preferably, it is soluble in water in any ratio. In addition, the water-soluble film-forming agent must be miscible with a physiological saline solution. Whether the film-forming agent is miscible with a physiological saline solution or not can be determined from its phase diagram. For example, 50 parts by weight of the film-forming agent or the bonding composition is mixed with 50 parts by weight of a physiological saline solution at room temperature. The uniformity of the resulting solution is determined. Accordingly, the miscibility of the film-forming agent with a physiological saline solution can be determined in a simple manner.
The film-forming agent may be selected from acrylates, acrylamides, crotonates, cinnamates and others that are soluble in water and miscible with a physiological saline solution. However, preferred are (meth)acrylate monomers as they are easy to polymerize and safe to bodies. More preferred are (meth)acrylate monomers having an increased number of polar atoms, except carbon and hydrogen, in the molecule. Most preferably, the value obtained by dividing the total sum of the number of the polar atoms and the atomic weight of each polar atom, by the molecular weight of the (meth)acrylate molecule is at least
Examples of (meth)acrylate compounds having a hydrophilic group such as a hydroxyl group, a carbonyl group, an amino group, an ammonium salt group, a phosphonium salt group, a sulfonic acid salt group, an ether bond, a cyclic ether group, an acyl group or the like in the molecule are given. They include, for example, hydroxyl group containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 1,3- and 2,3-dihydroxypropyl (meth)acrylates, pentaerythritol mono- and di(meth)acrylates, dipentaerythritol mono-, di- and tri(meth)acrylates and xylitol mono- and di(meth)acrylates; amides such as (meth)acrylamide, 2-hydroxyethyl(meth)acrylamide, N,N-bis(2-hydroxyethyl)(meth)acrylamide, N-alkyl-N-hydroxyethyl(meth)acrylamides, 2- and 3-hydroxypropyl(meth)acrylamides and methacrylamidopropyltrimethylammonium chloride; glycol (meth)acrylates such as diethylene glycol mono(meth)acrylate, triethylene glycol mono(meth)acrylate, polyethylene glycol(400) mono(meth)acrylate and methoxypolyethylene glycol (meth)acrylate; other hydrophilic monomers such as dimethylaminoethyl (meth)acrylate, 2-trimethylammoniumethyl (meth)acrylate hydrochloride, pyrrolidone methacrylate and sorbitol (meth)acrylate. One or more of these may be used either singly or in combination. Preferred are hydroxyalkyl (meth)acrylates as hydrophilic monomers. Particularly preferred is 2-hydroxyethyl methacrylate. Preferably, it is the essential ingredient of the film-forming agent, for example, accounting for at least 50% by weight of the agent.
In case where two or more monomers are combined to give the film-forming agent, a plurality of the hydrophilic monomers as mentioned above can be selected and combined. If desired, any other monomers may be added thereto, as long as they satisfy the requirement that a mixture of 50 parts of the mixed film-forming agent and 50 parts of a physiological saline solution is a uniform solution. The additional monomers may be monofunctional (meth)acrylates, including, for example, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, glycidyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, allyl (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxytetraethylene glycol (meth)acrylate, 2-ethoxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxyhexaethylene glycol (meth)acrylate, dicyclopentadiene-dimethanol-di(meth)acrylate, isobornyl (meth)acrylate, phenyl (meth)acrylate, caprolactone-modified tetrahydrofurfuryl (meth)acrylate, caprolactone-modified dipentaerythritol (meth)acrylate and caprolactone-modified 2-hydroxyethyl (meth)acrylate.
It is possible to use polyfunctional (meth)acrylates, including, for example, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 2-hydroxypropyl-1,3-di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentylglycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, bisphenol A di(meth)acrylate, bisphenol A-glycidyl di(meth)acrylate, ethylene oxide-modified bisphenol A di(meth)acrylate, ethylene oxide-modified bisphenol A-glycidyl di(meth)acrylate, 2,2-bis(4-methacryloxypropoxyphenyl)propane, 7,7,9-trimethyl-4,13 -dioxa-3,14-dioxo-5,12-diazahexadecane-1,16-diol di(meth)acrylate, neopentylglycol hydroxypivalate di(meth)acrylate, caprolactone-modified neopentylglycol hydroxypivalate di(meth)acrylate, trimethylolethane di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolmethane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, reaction product of 3-chloro-2-hydroxypropyl (meth)acrylate and methylcyclohexane diisocyanate, reaction product of 2-hydroxypropyl (meth)acrylate and methylcyclohexane diisocyanate, reaction product of 2-hydroxypropyl (meth)acrylate and methylene bis(4-cyclohexyl isocyanate), reaction product of 2-hydroxypropyl (meth)acrylate and trimethylhexamethylene diisocyanate, reaction product of 2-hydroxyethyl (meth)acrylate and isophorone diisocyanate, reaction product of 3-chloro-2-hydroxypropyl (meth)acrylate and isophorone diisocyanate.
The film-forming agent may be a mixture that comprises, as the essential ingredient, any of the above-mentioned hydrophilic monomers and contains any of the above-mentioned monofunctional (meth)acrylates and/or polyfunctional (meth)acrylates. Preferably, the mixture contains a polyfunctional (meth)acrylate, to ensure better bonding durability of the bonding layer. In this case, the blend ratio of the monofunctional (meth)acrylate and/or the polyfunctional (meth)acrylate may fall between 0.1 and 40 parts by weight, preferably between 1 and 20 parts by weight to 100 parts by weight of the film-forming agent. The blend ratio includes all values therebetween, and especially including 0.2, 0.5, 1, 1.5, 2, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, 30, 32.5, 35 and 37.5% per 100 parts by weight of the film-forming agent. The amount of the film-forming agent may vary within the range suitable for attaining the object of the invention. Preferably, it falls between 10 and 98% by weight, more preferably between 15 and 96% by weight, even more preferably between 20 and 90% by weight of the bonding composition. The amount of film-forming agent includes all values therebetween, especially including 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, and 95% by weight of the bonding composition.
The bonding composition of the present invention must contain water to ensure the bonding ability. Preferably, water is in the products of the composition produced and distributed by manufacturers, but may be added thereto by dentists and other users after they have bought the products or just before they use them. Also preferably, the water is pure water such as distilled water, deionized water, water purified through reverse osmosis or the like. The used water may be tap water when the composition is used in field hospitals. It may be electrically pre-treated in an electric field. The amount of water may vary within the range suitable for attaining the object of the invention, but preferably falls between 1 and 80% by weight, more preferably between 10 and 70% by weight of the composition. The amount of water includes all values therebetween, especially including 2, 3, 4, 5, 7.5, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, 30, 32.5, 35, 40, 45, 50, 55, 60, 65, 70 and 75% by weight.
If desired, a water-soluble, volatile organic solvent may be added to the bonding composition of the present invention to augment the solvent effect of water in the composition. The organic solvent includes, for example, acetone, methyl ethyl ketone, methanol, ethanol and isopropyl alcohol. One or more of these organic solvents may be used either singly or in combination. Ethanol is particularly preferred. The amount of the organic solvent preferably falls between 1 and 50% by weight, more preferably between 2 and 40% by weight of the bonding composition. The amount of organic solvent includes all values therebetween, especially including 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37 and 39% by weight.
The bonding composition of the invention contains a curing agent. The curing agent may be a polymerization initiator generally used in ordinary bonding compositions, or may be a mixture of a polymerization initiator and a polymerization promoter. The polymerization initiator may be any of photo-polymerization initiators, thermal polymerization initiators and their mixtures. Preferably, it is soluble in water. However, if a polymerization initiator is poorly soluble in water, the bonding composition is preferably so conditioned that it does not deposit the initiator on the tooth tissue or does not precipitate the initiator while it is stored.
Preferred photopolymerization initiators are a-diketone compounds, ketal compounds, anthraquinone compounds, thioxanthone compounds, benzoin alkyl ether compounds and acylphosphine oxide compounds.
The xcex1-diketone compounds include, for example, camphorquinone, benzil, diacetyl, acenaphthenequinone and 9,10-phenanthraquinone. The ketal compounds include, for example, benzyldimethyl ketal, benzyldiethyl ketal, benzyldi(xcex2-phenylethyl) ketal and benzyldi(2-methoxyethyl) ketal. The anthraquinone compounds include, for example, anthraquinone, xcex2-methylanthraquinone, and xcex2-ethylanthraquinone. The thioxanthone compounds include, for example, 2-ethythioxanthone, 2-chlorothioxanthone and 2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-N,N,N-trimethyl-1-propaneaminium chloride. The benzoin alkyl ether compounds include, for example, benzoin methyl ether, benzoin ethyl ether and benzoin propyl ether. The acylphosphine oxide compounds include, for example, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, 2,6-dimethylbenzoyldiphenylphosphine oxide and 2,6-dimethoxybenzoyldiphenylphosphine oxide. Particularly preferred photopolymerization initiators are camphorquinone, benzil and 2,4,6-trimethylbenzoyldiphenylphosphine oxide.
Examples of thermal polymerization initiators are organic peroxides, including, for example, diacyl peroxides, peroxy esters, dialkyl peroxides, peroxy ketals, ketone peroxides and hydroperoxides. The diacyl peroxides include, for example, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide and m-toluoyl peroxide. The peroxy esters includes, for example, t-butylperoxy benzoate, 2,5-dimethyl-2,5-bis(benzoylperoxy)hexane and t-butylperoxy-2-ethylhexanoate. The dialkyl peroxides include, for example, dicumyl peroxide, di-t-butylperoxide and lauroyl peroxide. The peroxy ketals include, for example, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. The ketone peroxides include, for example, methyl ethyl ketone peroxide. The hydroperoxides include, for example, cumemehydroperoxide and t-butylhydroperoxide. Particularly preferred is benzoyl peroxide as a thermal polymerization initiator.
Preferred polymerization promoters are amines, sulfinic acids and their salts. The amines are, for example, aromatic tertiary amines, and aliphatic tertiary amines. The aromatic tertiary amines include, for example, N,N-dimethylaniline, N,N-dimethyl-p-toluidine, N,N-diethyl-p-toluidine, N,N-dimethyl-3,5-dimethylaniline, N,N-dimethyl-4-ethylaniline, N,N-dimethyl-4-t-butylaniline, N,N-bis(2-hydroxyethyl)p-toluidine, N,N-bis(2-hydroxyethyl)-3,5-dimethylaniline, N,N-bis(2-hydroxyethyl)-3,4-dimethylaniline, N,N-bis(2-hydroxyethyl)-4-ethylaniline, N,N-bis(2-hydroxyethyl)-4-t-butylaniline, N,N-bis(2-hydroxyethyl)-3,5-di-t-butylaniline, ethyl 4-dimethylaminobenzoate, n-butoxyethyl 4-dimethylaminobenzoate, (2-methacryloyloxy)ethyl 4dimethylaminobenzoate and 4-dimethylaminobenzophenone. The aliphatic tertiary amines include, for example, trimethylamine, triethylamine, N-methyldiethanolamine, N-ethyldiethanolamine, N-n-butyldiethanolamine, N-lauryldiethanolamine, triethanolamine, (2-dimethylamino)ethyl methacrylate, N-methyldiethanolamine dimethacrylate, N-ethyldiethanolamine dimethacrylate, triethanolamine monomethacrylate, triethanolamine dimethacrylate, and triethanolamine trimethacrylate.
The sulfinic acids and their salts include, for example, benzenesulfinic acid, sodium benzenesulfinate, potassium benzenesulfinate, calcium benzenesulfinate, lithium benzenesulfinate, toluenesulfinic acid, sodium toluenesulfinate, potassium toluenesulfinate, calcium toluenesulfinate, lithium toluenesulfinate, 2,4,6-trimethylbenzenesulfinic acid, sodium 2,4,6-trimethylbenzenesulfinate, potassium 2,4,6-trimethylbenzenesulfinate, 2,4,6-triethylbenzenesulfinic acid, sodium 2,4,6-triethylbenzenesulfinate, potassium 2,4,6-triethylbenzenesulfinate, 2,4,6-triisopropylbenzenesulfinic acid, sodium 2,4,6-triisopropylbenzenesulfinate and potassium 2,4,6-triisopropylbenzenesulfinate. Preferred are amines as polymerization promoters such as N,N-bis(2-hydroxyethyl)-p-toluidine, ethyl 4-dimethylaminobenzoate and n-butoxyethyl 4-dimethylaminobenzoate; and salts of sulfinic acids such as sodium benzenesulfinate, sodium toluenesulfinate and sodium 2,4,6-triisopropylbenzenesulfinate.
The amount of the curing agent may vary within the range suitable for attaining the object of the invention. It may fall generally between 0.05 and 20% by weight, but preferably between 0.1 and 20% by weight, more preferably between 1 and 10% by weight of the bonding composition. The amount of the curing agent includes all values therebetween, especially including 0.15, 0.25, 0.35, 0.45, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 and 19% by weight.
The method of adding the curing agent to the bonding composition of the invention is not specifically limited, as long as it attains the object of the invention. For example, in case where the bonding composition is to be cured by photopolymerization, it may be packed a single package containing both a photopolymerization initiator and a polymerization promoter. However, if it contains a thermal polymerization initiator and a polymerization promoter, it may be divided into two or more packages so that a peroxide and an amine or a peroxide and a salt of a sulfinic acid are not in one and the same package, in order to ensure the storage stability of the composition. Two or more packages separately containing the constituent ingredients of the bonding composition may be blended before use. If the bonding composition is applied twice or more to a surface, another method may be employed. For example, one part of the composition containing a polymerization initiator is first applied to an object, and thereafter another part containing a polymerization promoter alone is applied thereto. This is for ensuring the migration and mixture of the ingredients of the curing agent on the surface of the object. The blend ratio of all the ingredients constituting the bonding composition applied to the object must satisfy the requirement of the invention.
As so mentioned hereinabove, the bonding composition of the invention contains a curing agent, and the curing agent may be a polymerization initiator alone, or a mixture of a polymerization initiator and a polymerization promoter. Preferably, the bonding composition is curable as rapidly as possible, since the thickness of the uncured surface layer of the composition applied to an object can be thinner. If the rapidly-curable composition is applied to an object to a predetermined level, its cured layer has the intended thickness, and the bonding interface is protected from oxygen that may act as a polymerization inhibitor. As a result, the bonding interface can be highly polymerized. Accordingly, the polymerization initiator enables easy and rapid polymerization of the bonding composition and ensures the storage stability thereof. The photopolymerization initiator gets energy, after having absorbed light forming active radicals. It is generally combined with a polymerization promoter that promotes the radical formation. Such photopolymerization initiator may be additionally combined with a thermal polymerization initiator, and the combination is effective in the invention. Preferably, the bonding composition of the invention is a photopolymerizable liquid bonding composition in the form of one single package containing all the necessary constituent ingredients therein. Though not intended, however, the active ingredients of the composition in such single packages may degrade or polymerize while stored. To prevent this, the constituent ingredients of the composition may be divided into two or more parts. The plural parts are separately packaged and stored in different packages. For their use, the plural parts taken out of the individual packages may be applied to one and the same object in sequence; or they may be blended into one mixture just before use.
Further preferably, the bonding composition of the present invention contains a form-retaining agent. The form-retaining layer acts to control the thickness of the bonding layer of the composition. Specifically, it reinforces the bonding layer and ensures the desired thickness of the bonding layer. Accordingly, inhibition of the polymerization in the bonding interface caused by oxygen in air can be prevented, and the bonding interface can be smoothly polymerized and cured to provide increased bonding strength. In general, the thickness of the bonding layer is controlled to fall between 20 and 300 micron, preferably between 20 and 100 micron. The thickness of the bonding layer includes all values therebetween, especially including 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260 and 280 micron.
For its morphology, the form-retaining agent is not specifically limited, and may be any of granular, tabular, sheet-like, fibrous or porous structure. In view of its handle ability, the form-retaining agent is preferably granular. Furthermore, a hydrophobic polymerizable composition, for example, a dental restorative composite resin is applied to the site to which the bonding composition is applied. Therefore, it is desirable that the form-retaining agent is treated, for example, with a silane coupling agent to improve its compatibility with composite resins. It is desirable that the particles of the form-retaining agent are not too fine to prevent an increase of the viscosity of the bonding composition. It is preferred that the particles of the form-retaining agent are large enough to allow for free movement of a liquid component along with them. However, they do not almost move by themselves to increase the viscosity of the bonding composition. For example, the form-retaining agent may have a mean particle size of from 1 to 300 micron, preferably from 3 to 100 microns, more preferably from 5 to 80 micron. The mean particle size includes all values therebetween, especially including 2, 3, 4, 6, 7, 8, 9, 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260 and 280 micron.
The form-retaining agent includes, for example, sand balloons, glass balloons, short glass fibers, pieces of hollow glass fibers, glass beads, glass powders, powders of various natural minerals, beads and flakes of various crosslinked polymers, organic/inorganic composite materials containing such inorganic substances and crosslinked polymers. Preferred form-retaining agents are crosslinked polymers because they do not solidify through precipitation or aggregation in the bonding composition and their specific gravity can be nearly the same as that of the bonding composition. The granular crosslinked polymers may be prepared, for example, through suspension or emulsion copolymerization of the above-mentioned monofunctional (meth)acrylates and polyfunctional (meth)acrylates. The crosslinked polymers may be swollen when mixed with the bonding composition comprising a polymerizable compound containing an acid group, a film-forming agent and an organic solvent. In general, the degree of swelling of the crosslinked polymers in those ingredients may be at most 100%.
The amount of the form-retaining agent that may be in the bonding composition of the present invention may fall between 0.5 and 20% by weight, preferably between 1 and 10% by weight. The amount of form-retaining agent includes all values therebetween, especially including 1.5, 2.5, 5, 7.5, 10, 12.5, 15 and 17.5% by weight.
The method to be employed herein for preventing the form-retaining agent from aggregating and solidifying in containers is not specifically limited. For example, hard balls, cylindrical or oval blocks may be put in the container with the bonding composition. The container is shaken to agitate the contents before the composition is used. In this method, the specific gravity of the form-retaining agent can be neglected without problem.
As mentioned above, the bonding composition preferably comprises from 1 to 50% by weight of a polymerizable compound having an acid group, from 10 to 96% by weight of a water-soluble film-forming agent, from 1 to 80% by weight of water, and from 0.1 to 20% by weight of a curing agent. More preferably the bonding composition comprises from 1 to 50% by weight of a polymerizable compound having an acid group, from 10 to 96% by weight of a water-soluble film-forming agent, from 1 to 80% by weight of water, from 0.1 to 20% by weight of a curing agent, and from 0.5 to 20% by weight of a form-retaining agent. If desired, additives generally used in ordinary bonding compositions, for example, pigments or dyes for differentiation, viscosity improvers for attaining optimum viscosity and others may be added to the bonding composition of the invention, as long as they do not have any negative influences on the composition.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.